CMOS image sensor belongs to the optoelectronic components. The manufacturing process of CMOS image sensor is compatible with the manufacturing process of integrated circuit. CMOS image sensor is better than the charge coupled device (“CCD”, hereinafter) in lower power consumption, faster, cheaper, larger bandwidth and so on. CMOS image sensor has become very popular in image sensor field. The driver circuit and the pixel are jointly integrated in CMOS image sensor, which simplified the design of hardware and reduced the consumption of the system power. CMOS image sensor can capture the signals of light and electrical simultaneously, and can further deal with the information of images in real time, whose speed is faster than that of CCD image sensor. Further more, CMOS image sensor has the characteristics of low cost, wide bandwidth, anti fuzzy, flexible access and the larger filling factor. Therefore, CMOS image sensor is widely used in the industrial automation control, the consumer electronics and other products, such as monitors, video communication products, toys and so on. The research and the development in CIS (CMOS image sensor) are to realize the multi-function and the intelligent with the advantage of system integration. The high frame rate of CIS can be achieved by the advantage of flexible access, i.e., it only reads the needed region of photo-surface. Moreover, the technologies of the wide dynamic range, the high resolution and the low noise are being developed.
FIG. 1 is the diagram of the 4 T active CMOS image sensor. There is a structure of shallow isolation trench around the active region of the P-type epitaxial layer. An N-type Doped Region 104 provided with photodiode is formed in the active region, which is located at the top of the active region. There is a P-well-region 107 in the active region, which is close to the N-type Doped Region 104. A Floated First N-type Region 109 and a Floated Second N-type Region 108 are formed at the top of P-well-region 107. A gate oxide and a gate which is disposed on the gate oxide are formed above the P-well-region which is between the first N-type Region 109 and the N-type Doped Region 104, consequently, a Transfer Transistor 110 is formed. There is a gate oxide and a gate which is disposed on the gate oxide are formed above the P-well-region which is between the first N-type Region 109 and the second N-type Region 108, consequently, a Reset Transistor 111 is formed. An amplifier transistor and a select transistor are formed in the active region. The N-well-region 109 is electrically connected to the gate of the amplifier transistor. The amplifier transistor and the select transistor are connected in series, and the two ends of the amplifier transistor are connected with the power supply voltage VDD and the select transistor severally. The second N-type Region 108 is in the potential of supply voltage. Ions are implanted into the exposed back of the P-type epitaxial layer. If the exposed back of the P-type epitaxial layer is regarded as the first surface in the ion implantation, it is defined that the closer the doped ions are from the front of the P-type epitaxial layer, the more deeply the doped ions implanted. As a reference, the implanted depth of implantation of the first doped ions is deeper than that of the second doped ions. Therefore, the first doped ions need more energy for support.
In order to remove the electronic of photodiode and the floating points, the transfer transistor and the reset transistor are switched on before lighting. When in lights, the charge will be generated in the N− of the photodiode, and Transfer Transistor 110 is switched off at this time. Then Transfer Transistor 110 is switched on. As there are some charges in the photodiode, the charge will be transported into the floating nodes. Then, the transfer transistor is switched off and waits for the next beam of light. Then the electrical signal of floating nodes is used to adjust the amplifier transistor. The reset transistor with a reset gate will reset the floating points to a reference voltage after read out. For CMOS image sensor of FSI (front side illumination), the light will pass through a dielectric layer and a metal stack layer, and this will reduce the efficiency of light. People had created a method for reducing the distance between light and the photodiode, i.e., the BSI-CIS (backside illuminated photo CMOS image sensor). The light can reach the photodiode directly. Therefore, the efficiency of light is improved, and the crosstalk is reduced caused by the reflecting light or the optical absorption of the metal layer and the dielectric layer. However, as the light gets into the backside of the CMOS image sensor directly. Therefore, there is no metal barrier blocking the light. In another word, the light will through the silicon substrate directly. That leads to the electron diffusion. The photoproduction electron will get into the substrate when the light is strong, and also will cause the crosstalk among the pixels. When the light reaches the substrate, which is the outside of the charge region of photodiode. The diffused electrons will be absorbed by the charge region in substrate. However, because of the irregularity of the electron diffusion, they can combine with the holes in the substrate, or walk for a while in the substrate. The electrons perhaps get into the other region of charge. Therefore, it causes a new crosstalk among the pixels. This is called the electric crosstalk. The electric crosstalk will also bring some false signals to pixels, and reducing the SNR (signal to noise ratio), which means the quality of image will be poor. When in the strong light, the problem of electric crosstalk will be very serious. The photoproduction electron, which is generated in the outside of charge region of the photodiode, will diffuse into the substrate, further, the electron which has been collected by the charge region will diffuse into the substrate once again. That causes some defects in imaging, just like a halo. The reason is that the number of the electrons that the pixels can accommodate is limited, and the P-N junction will come into balance from reverse biased by collecting enough electrons. The excess electrons will overflow and diffusion into the substrate, wherein most of them will be absorbed by the neighboring pixels, then the brightness of surrounding pixel will be increased, so the halo is formed, and the imaging quality of CMOS image sensor is affected.
Chinese Patent (CN 101752394 A) has disclosed a method of manufacturing the back illuminated CMOS image sensor. Firstly, an assembly of back illuminated type image sensor is formed in the front of the array. Then, the doped layer is implanted into the backside of the CMOS image sensor. The doped layer can establish the gradient of dopant, and promote the migration of the photo-generated electrons migrating to the front of the array. FIG. 2 is the schematic diagram of the BSI (backside illumination) structure, which has been treated by the latter process. The photodiode which is doped at the back is annealed. The untreated region can capture the diffusion electrons in the substrate, for inhibiting the halo. The above method can utilize the doped layer which is not activated to capture the electrons diffused into the substrate; the phenomena of halo in strong light can be improved. However, the unactivated doped layer is unstable, and the number of unactivated electron is restricted. Hence, the unactivated region will lose the ability of capturing the electron when exposed in lights for a short time, and the electrons will spread to other regions. Consequently, the image quality of CMOS image sensor will be damaged.
Chinese Patent (CN 103139497A) has disclosed an active pixel of a complementary metal oxide semiconductor (CMOS) image sensor. The active pixel at least comprises a photosensitive component which is placed inside a semi-conductor substrate, a reset transistor and a source following transistor which are connected with the photosensitive component, a switch transistor, and a row positioning line. The pixel photosensitive component of the image sensor comprises two photosensitive regions, namely a low dose foreign ion injection region, and a high dose foreign ion injection region near the reset transistor. When in low illumination, photo-electricity charge generated in the photosensitive component is only collected in the high dose foreign ion injection region. The grain of photo-electricity transition is high, and sensitivity of the sensor is high. When in high illumination, photo-electricity charge generated in the photosensitive component is collected in the whole photosensitive component. Therefore, the active pixel can effectively improve the luminous sensitivity of the image sensor in the low illumination, and the sensor collects more real object detail information when in the low illumination.
The said invention comprises two ion implanting regions whose dosage of doped ions is different from each other. The image quality in strong light or weak light can be improved. However, the substrate in the above patent is a traditional substrate. Therefore, in the process of image acquisition, when the light is strong, there will be the photoelectric electrons diffused into the substrate, it causes the electric crosstalk among the pixels, consequently, the imaging quality of image is affected.